Soil moisture content measurement system and method

ABSTRACT

The invention provides a soil moisture content measurement system comprising a porous plate arranged to support a soil sample; a hanging water tube extending downwardly from the porous plate, the tube arranged to convey liquid toward and away from the porous plate; a measuring capillary tube in connection with the hanging water tube, the measuring capillary tube arranged to convey liquid toward and away from the hanging water tube; measurement apparatus configured to measure the movement of liquid within the measuring capillary tube; and a data memory configured to receive and store data from the measurement apparatus representing liquid movement measurements within the measuring capillary tube. The invention also provides a related method of measuring soil moisture content.

RELATED APPLICATIONS

This application is a National Stage application of PCT/NZ03/00122 filedJun. 17, 2003, which claims priority from New Zealand Patent ApplicationNo. 519609 filed Jun. 17, 2002.

FIELD OF INVENTION

The invention relates to an automated system and method for obtainingsoil moisture content data to estimate moisture release curves,particularly designed to obtain drying and wetting water release curvesfor an undisturbed or remoulded soil sample.

BACKGROUND TO INVENTION

One of the most important soil physical relationships requiringestimation is the amount of water available in the soil represented by awater retention curve. A water retention curve represents therelationship between soil water tension (matric potential) and soilwater content of the soil. Many models have been developed to estimatethe amount of water available in soil from small farmlands tolarge-scale catchments. The soil water retention curve plays a majorrole in many of these models. Studies of water balance and runoffgeneration leading to sediment production require the calculation of asoil water retention curve at the lower end (0 to 100 cm soil watertension).

The most popular and only reliable method to obtain the soil waterretention curve at the lower tension is a tension plate with a hangingwater column. This method has been in use for over 70 years. A tensionplate is made from a porous material such as ceramic or a sand bed.Since larger pore space dominates the soil water characteristics at thelower tension range, tests must be carried out with either minimum or nodisturbance to the soil sample.

To use the tension table, the soil sample must first be saturated.During saturation, the weight of the soil sample is checked regularly tofind the equilibrium point. Once the saturation process is completed,different tensions are applied to the soil sample, which sits on thetension plate, by the hanging water column. In order to find theequilibrium point for each applied tension, the water meniscus in thehanging water column is monitored regularly. This is a very timeconsuming task. Once the soil sample reaches equilibrium, it istransferred to a weighing apparatus at the end of each tension step toestimate the moisture content.

The main disadvantages of this method are:

-   -   1. Disturbance to the fragile soil matrix (larger pore        structure) due to regular handling of the soil sample.    -   2. Inaccurate soil moisture estimation due to sample handling.    -   3. High labour intensity. Regular supervision is necessary to        determine the equilibrium points.

It would be useful to at least partially automate the test procedure toeliminate or reduce the drawbacks. An added advantage of an automatedsystem could be the ability to produce accurate outflow data to estimatethe unsaturated hydraulic conductivity.

SUMMARY OF INVENTION

In broad terms in one form the invention comprises a soil moisturecontent measurement system comprising a porous plate arranged to supporta soil sample; a hanging water tube extending downwardly from the porousplate, the tube arranged to convey liquid toward and away from theporous plate; a measuring capillary tube in connection with the hangingwater tube, the measuring capillary tube arranged to convey liquidtoward and away from the hanging water tube, the measuring capillarytube arranged to be raised and/or lowered with respect to the soilsample; measurement apparatus configured to measure the movement ofliquid within the measuring capillary tube; and a data memory configuredto receive and store data from the measurement apparatus representingliquid movement measurements within the measuring capillary tube.

In broad terms in another form the invention comprises a method ofmeasuring a moisture retention curve of a soil sample comprising thesteps of supporting a soil sample on a porous plate; positioning theheight of a measuring capillary tube with respect to the porous plate toenable liquid to be conveyed between the measuring capillary tube andthe porous plate; positioning a hanging water tube to convey liquidbetween the porous plate and the measuring capillary tube; performing atleast one purging cycle in which liquid is introduced into the measuringcapillary tube and the soil sample; performing at least one drying cyclein which the measuring capillary tube is substantially emptied ofliquid, liquid is permitted to travel from the soil sample through theporous plate to the measuring capillary tube, and the movement of liquidwithin the measuring capillary tube is measured; storing in computermemory data representing liquid movement measurements within themeasuring capillary tube; and calculating the moisture retention curvefrom the data representing liquid movement measurements.

BRIEF DESCRIPTION OF THE FIGURES

Preferred forms of the moisture release curve calculation system andmethod will now be described with reference to the accompanying Figuresin which:

FIG. 1 is a preferred form system of the invention;

FIG. 2 shows a hardware user interface apparatus forming part of thesystem of FIG. 1;

FIG. 3 shows a software user interface forming part of the system ofFIG. 1;

FIG. 4 shows a set up window from the interface of FIG. 3;

FIG. 5 shows a collect data window from the interface of FIG. 3;

FIG. 6 shows a status panel forming part of the interface of FIG. 3;

FIG. 7 shows a manual control button forming part of the interface ofFIG. 3;

FIG. 8 illustrates sample raw data from the system of FIG. 1; and

FIG. 9 shows a typical soil moisture release curve.

DETAILED DESCRIPTION OF PREFERRED FORMS

FIG. 1 shows a schematic representation of one form of the invention 10.A soil sample retaining ring 20 is positioned on a porous plate 40 whichis rigidly connected to a sealed water reservoir 35. The porous plate 40and the water reservoir beneath it, rests on the structural support orbase 30, as a single unit. The porous plate is constructed of a suitablematerial that is permeable to liquid such as water, and is preferably ofa high flow type with an air entry value of 0.5 bar. The soil sampleretaining ring 20 is preferably supported on the porous plate 40, andthe join between the soil sample retaining ring 20 and porous plate 40is coated with a suitable water impermeable material such as silicongrease in order to ensure a proper seal between the soil sampleretaining ring and the porous plate.

The retaining ring 20 contains the undisturbed or remoulded soil sample50 for which a moisture release curve is desired. Inside the sealedwater reservoir 35 a channel is formed in a shape of a spiral. Extendingdownwardly from one end of the spiralled channel in the sealed waterreservoir below the porous plate 40 is a hanging water tube 60. Thistube is arranged to convey liquid, for example water, upwardly towardand through the porous plate 40 where required and to convey water awayfrom the porous plate 40 where required. An air valve 70 fitted at theother end of the spiral shaped channel in the water reservoir releasesany air trapped in the tubes through the air valve 70 during a purgingcycle as described below.

A water receptacle or tank 100 containing a liquid such as water 110supplies de-aired water to the rest of the system 10 through a supplytube 120. This de-aired water 110 is used to fill the hanging water tube60 and a measuring capillary tube described below, which saturates thesoil sample 50. The water tank 100 is preferably elevated with respectto the soil sample 50 to exert sufficient pressure difference to forcethe water from the tank 100 through the sealed water reservoir 35 andporous plate 40 to the soil sample. The water supply tube 120 ispreferably fitted with a suitable tank valve 130 to control the flow ofwater exiting the tank 100 to the rest of the system.

The hanging water tube 60 is also fitted with a suitable sample valve140 to control the flow of water from the water tank 100 travelling intoand up the hanging water tube 60, and to control the flow of waterexiting the tube 60. A drain valve 150 is arranged to control the flowof water out of the supply tube 120 and the hanging water tube 60.

A measuring capillary tube 160 is connected to the ends of the hangingwater tube 60 and the supply tube 120. The horizontal measuring tube 160is vertically positioned with respect to the soil sample 50 by adistance “d” in order to apply tension to the soil sample 50. It isenvisaged that the distance “d” can be varied in order to alter theapplied soil water tension on the soil sample 50. The distance “d” couldbe varied by enabling the measuring tube 160 to be raised and loweredwith respect to the soil sample 50 using a stepper motor (not shown) inorder to apply different soil water tensions to the soil sample.

The measuring capillary tube 160 is preferably substantially horizontaland parallel to the porous plate 40 and is fitted with measurementapparatus to measure the displacement of water along the tube 160 ineither direction. It will be appreciated that the measuring capillarytube could be positioned substantially vertically or alternatively couldbe positioned at any angle to the horizontal.

In one form, the measurement apparatus includes a series of infraredemitters 165 positioned on one side of the measurement tube 160,together with a series of corresponding infrared detectors 170. Theemitters 165 and detectors 170 are preferably arranged as correspondingpairs. Modulated infrared beams at 40 kHz are transmitted from theinfrared emitters 165 to the infrared detectors 170 through themeasurement tube 160.

It is envisaged that part of the measuring tube 160 will contain waterand that a meniscus appears at the intersection between the part of thetube 160 filled with water and the part of the tube that does notcontain water. The meniscus will travel along the measuring tube 160 aswater enters or exits the tube 160.

Each pair of emitters and detectors preferably defines a segment of themeasuring tube 160; the amount of water in each segment ispre-determined. Each segment could hold, for example, 0.1 ml of water.The meniscus described above will obscure the infrared beams as itpasses between an emitter/detector pair and so indicate the direction ofwater movement and the number of water filled segments and hence volumein the measuring tube 160.

The horizontal measuring tube can be withdrawn for cleaning or can bereplaced with a smaller diameter measuring tube 160 to increase theresolution as desired.

The system 10 further comprises a hardware user interface 180, which isconfigured to receive, process and store data from the measuring tube160 and to control the rest of the system according to instructions fromthe user interface 180. The device includes a data memory configured tostore data and could have an associated microprocessor ormicrocontroller. The hardware user interface 180 includes a suitabledata port to which a personal computer, workstation, or otherprogrammable device 190 can be connected. The computer 190, runningappropriate software, sends the required configuration to the hardwareuser interface to perform the test. The computer 190 is also configuredto receive data stored in the micro controller and to process and togenerate a series of moisture release curves for the soil sample 50.

FIG. 2 shows the front panel of one form of the hardware user interface180. The hardware is preferably connected to a power supply using a12-volt line.

The hardware user interface 180 could include a pilot light 202 showingthe status and current operation of the system, and an RS232 serial port204 for connection to a personal computer or workstation. The hardwareuser interface could also be provided with a graphical liquid crystaldisplay (GLCD) 206 for displaying instructions, current progress of thesaturation wetting/drying cycles in graphical form as water in/outagainst time, and program details.

The apparatus also includes several button controls, for example ‘Bypassthe current tension step’ 208, ‘Start the test’ 210, and ‘activate theDisplay’ 212. The functions of these controls are described below.

The display 206 on the hardware user interface 180 could further includea series of LED displays 214. The number of LED displays illuminatedindicates the number of water segments in the measuring tube 160 fromFIG. 1 that are full of water. For example, if 4 LED displays areilluminated, then 4 segments in the measuring tube 160 are full ofwater, with each segment containing approximately 0.1 ml of water.

In order to obtain drying and wetting moisture release curves of a soilsample, the system 10 from FIG. 1 is placed through one or more purging,drying, and wetting cycles.

The intention of the purging cycle is to remove air bubbles from thetubes 60, 120, 160, the porous plate 40, and the water reservoir 35.Referring to FIG. 1, the distance “d” is minimised by, for example,raising the measuring tube 160 to an upper limit. The valves 140 & 150are closed and the valve 130 is opened to fill the measuring tube 160with de-aired water from tank 100. Then the valves 140 and 70 are openedand valve 130 closed, allowing water and trapped air bubbles in thetubes to flow through the tube 60 along the spiralled water channel inthe sealed water reservoir 35 to escape through the air valve 70.

It is envisaged that this purging process be repeated 20 times or in anycase enough times so that the water volume through the system isreplaced in all the tubes.

Following the purging cycle, a drying cycle imposes a tension on thesoil sample 50 by lowering the measuring tube 160 to a heightcorresponding to the tension required to remove water from the soilsample. The measuring tube 160 is first emptied, by opening the valve150 leaving all other valves closed. Valve 150 is closed as soon as thewater meniscus inside the measuring tube 160 reaches the first infraredsensor at the right end of the measuring tube 160. With all other valvesclosed, the valve 140 is then opened to enable water released from thesoil 50 to pass through the porous plate 40 down the hanging water tube60 to the measuring tube 160.

As the water passes along the measuring tube 160, the water meniscuswill trigger the emitter/detector pairs positioned along the measuringtube 160 and in this way, the direction of the water movement and themagnitude of the displacement of water along the measuring tube 160 ismeasured.

Water in the measuring tube 160 automatically drains from the tube whenall segments in the measuring tube 160 are full of water. Computer 190programmed by the user preferably controls all the activities of thesystem 10. It decides when to empty the measuring tube 160 by countingthe number of active infrared receivers in the measuring tube. It drainsthe tube as the number of filled segments reaches a predefined maximum,for example 6, during a drying or purging cycle. It re-fills themeasuring tube as the number of active segments reaches zero during awetting cycle.

Computer 190 records the active segment number in real time as the watermeniscus passes between each infrared beam. In this way, the amount ofwater taken up by the soil sample during a wetting cycle and the amountof water released by the soil sample 50 during drying can be estimated.The computer estimates the time elapsed since the water meniscus in themeasuring tube 160 passes a segment and compares it with thepre-determined time limit. In one form, a user could specify apredetermined time limit for the water meniscus in the measuring tube160 to move between two segments for each applied soil water tension.The time limit is the maximum time allowed for the water meniscus totravel between two segments under a given tension. If no water movementis detected in between two infrared beams in the measuring tube 160during this predetermined time limit, the user could assume that thesoil sample has reached equilibrium under the current tension.

Referring to FIG. 2, the user could press the Bypass button 208 to moveonto the next cycle should the user decide that the predetermined timelimit already programmed is too long. Alternatively, the system could beconfigured to move to the next cycle automatically as soon as thespecified equilibrium time limit has been reached.

Once the drying cycle has been completed, a wetting cycle then starts(if programmed by the user) that enables the sample to absorb water fromthe measuring tube 160. The valves 140 and 150 are closed and the valve130 opened to fill the measuring tube 160 with water 110 from the tank100. The valve 130 is then closed and the valve 140 opened to enable thesoil sample to take up water. As water travels along the measuring tube160 and up the hanging water tube 60, the meniscus in the measuring tube160 will travel along the measuring tube 160 and the movement of thismeniscus will be tracked by the emitter/detector pairs. Once themeasuring tube 160 is empty, the valve 140 is closed and the valve 130opened to refill the measuring tube 160. The valve 130 is then closedand the valve 140 opened to resume water uptake by the soil sample 50.The user preferably sets a predetermined time limit. If no movement ismeasured along the measuring tube 160 during this predetermined timeperiod, it is assumed that the soil 50 has reached equilibrium.Alternatively, the user could press the Bypass button 208 to completethe wetting cycle.

Computer 190 is preferably connected to the serial port 204 of theinterface 180 to configure the system 10. Software running on thepersonal computer 190 provides a graphical user interface to control allthe valves and movement of the measuring tube.

FIG. 3 illustrates a preferred form graphical user interface 180 ofsoftware installed and operating on the computer 190. In use, thecomputer 190 is connected through a serial port to the hardwareinterface 180. Clicking on the Set up Ports and Files button 302 bringsup the window shown in FIG. 4. The select port button enables a user toselect the COM port on the computer and to specify the appropriate baudrate. The user could select, for example, a baud rate of 9,600.

Once the port is set up, Wake Up J&J button 304 is selected by the userto start data communication between the computer 190 and the hardwareinterface 180. Data are retrieved from the interface 180 in order topopulate the Wetting and Drying Cycles panel 306 and the SuctionEquilibrium time limits panel 308 that represent previously programmedvalues.

Using Wetting and Drying Cycles panel 310, the user specifies newparameters for a soil sample, using the previous values in panel 308 asa guide. In panel 460, the user may specify the number of drying andwetting cycles, the number of suction steps, and the time limit for thesaturation process. Selecting the number ‘2’ will result in two dryingcycles and two wetting cycles for example. The user may specify thenumber of suction steps up to a maximum of 10. The suction steps arealso referred to as tension steps. The user may also specify apredetermined time limit for the saturation process. A time of 1 minutemeans that the saturation process will be terminated if the soil sampledoes not cause one segment of movement (0.1 ml) in the measuring tube160 during a 1-minute period. The user can also specify the soil sampleheight.

In panel 312, the user can specify, for each suction or tension step, atension value and a time limit. The tension value represents thedistance “d” between the soil sample 50 and the measuring tube 160. Inthis preferred form, the resolution of the suction is 1 mm and themaximum suction is 1000 mm. It is envisaged that the apparatus 180controls distance “d” and this distance “d” is adjusted according to thesuction steps specified in the user-entered program.

A four-phase unipolar stepper motor preferably controls the movement ofthe hanging water column, which then varies the suction applied to thesoil sample.

The user is also able to specify three different time limits todetermine the equilibrium status for a saturating, drying, and wettingcycle under a given tension. If the soil sample does not take onesegment of water during the wetting process, or remove one segment ofwater during the drying process within the specified time limit for aparticular step, then the system assumes that the soil sample has cometo equilibrium under that tension value.

Once the user has entered the required data into the panels 310 and 312,data representing these parameters are transferred to the hardwareinterface 180, and the computer 190 is disconnected. After disconnectingthe computer 190 from the interface 180, the system 10 undergoes severalpurging cycles to remove air bubbles from the system. The display 206 onthe hardware interface 180 tells the user that the system is beingpurged. The display could also show the user the purging cycle numbercurrently being performed by the system.

Once the system is purged, the soil sample is placed on the porous plate40 and the Start button 210 pressed to initiate the first drying cycleand/or the whole experiment. The display could indicate to the userwhether the system is undergoing a drying or a wetting cycle, theapplied tension in centimetres applied to the soil sample 50, the volumeof water taken up or expelled from the soil sample in segments, and thetotal time elapsed to take or expel these segments.

The drying or wetting cycle continues until the equilibrium time limitspecified by the user has been reached or the system has been bypassedwith the user pressing the Bypass button 208.

The display is preferably a graphical liquid crystal display (GLCD) 206programmed to turn off after a few minutes but restarted by the userpressing the Display button 212. The Display button 212 enables a userto select the required information window. For example, a single pressof the Display button 212 will show the progress of the current suctionstep, pressing it twice will show the overall progress of the currentcycle. Three presses of the Display button 212 will show the programdetails.

On completion, the display 206 reveals a test completion message. Theuser reconnects the computer 190 to the apparatus 180 and the “Wake-UpJ&J” button 304 selected by the user.

Referring to FIG. 3, the user presses the “Get Data” button 314, whichpresents to the user the window shown in FIG. 5. The user then selectsthe “Get Data” button to start collecting data from the apparatus 180.The panel shows various text messages to the user guiding the userthrough the process of data collection.

Once the user has collected the data from the micro controller, the userselects the “Set Up Ports & Files” button 302 and selects the “SelectFile” button shown in FIG. 4. The user may then specify a file name bywhich to index the data retrieved from the apparatus 180 using the “SaveData” button.

The downloaded data is preferably saved to an EXCEL file and the finalmoisture release curves are automatically plotted.

Referring to FIG. 3, panel 308 could include a “Current Progress” buttonClicking this button presents to a user a status panel such as thatshown in FIG. 6. This window displays the current progress of the systemsuch as the number of cycles being completed, current tension, and thetime spent waiting for the next water segment to be filled or emptiedsince the last segment was detected.

As an alternative to a “Current Progress” button the display couldperiodically toggle between screens.

Referring to FIG. 3, the panel 308 may also include a manual controlbutton 316. Clicking this button presents to a user a control panel suchas that shown in FIG. 7 enabling a user to manually control parametersof the system, for example close or open the sample valve, close or openthe air valve, close or open the drain valve, close or open the tankvalve. The user may also return the measuring tube 160 or rack to adefault position or may raise or lower the capillary tube.

FIG. 8 shows sample raw data transferred from the apparatus 180 to thecomputer 190. The data could include program parameters, as a series ofdescriptive text strings followed by parameter values, and status data.Each data string could include, for example, hour, minute, second, cycledescription (for example saturation/purging cycle, drying cycle orwetting cycle), tension value, cycle number, and cumulative volume as anumber of segments.

The software running on the computer 190 could also be configured togenerate a series of graphs representing the data retrieved from theapparatus 180.

FIG. 9 shows a sample moisture release curve 900 produced from theoutflow data downloaded in FIG. 8. The program parameters 902 and 904are also written when the data file is saved.

The moisture release curve calculation system and method of theinvention has the advantage that data is stored automatically in aprogrammable computing device, for example a micro controller EEPROMnon-volatile memory. Data will remain in the memory of the apparatus 180after the power is switched off and can be transferred to the computer190 at any time. The major advantage of this system and method is thatthe soil sample 50 is not disturbed during measurement resulting ingreater accuracy.

The foregoing describes the invention including preferred forms thereof.Alterations and modifications as will be obvious to those skilled in theart are intended to be incorporated within the scope hereof as definedby the accompanying claims.

1. A soil moisture content measurement system comprising: a porous platearranged to support a soil sample; a hanging water tube extendingdownwardly from the porous plate, the tube arranged to convey liquidtoward and away from the porous plate; a measuring capillary tube inconnection with the hanging water tube, the measuring capillary tubearranged to convey liquid toward and away from the hanging water tube,the measuring capillary tube arranged to be raised and/or lowered withrespect to the soil sample; measurement apparatus configured to measurethe movement of liquid within the measuring capillary tube; and a datamemory configured to receive and store data from the measurementapparatus representing liquid movement measurements within the measuringcapillary tube.
 2. A soil moisture content measurement system as claimedin claim 1 further comprising a microcontroller associated with the datamemory, the microcontroller configured to monitor the movement of liquidin the measuring capillary tube and control movement of liquid betweenthe hanging water tube and the measuring capillary tube by raising orlowering the measuring capillary tube with respect to the soil sample.3. A soil moisture content measurement system as claimed in claim 2wherein the data memory and microcontroller are connectable to acomputer device.
 4. A soil moisture content measurement system asclaimed in claim 3 wherein data stored in the data memory istransferable to the computer device.
 5. A soil moisture contentmeasurement system as claimed in claim 3 wherein commands aretransferred from the computer device to the microcontroller.
 6. A soilmoisture content measurement system as claimed in claim 1 wherein theporous plate is elevated with respect to the measuring capillary tube.7. A soil moisture content measurement system as claimed in claim 1further comprising: a liquid receptacle elevated with respect to thehanging water tube and/or the measuring capillary tube; and a supplytube extending downwardly from the liquid receptacle, the tube inconnection with and arranged to convey liquid to the hanging water tubeand/or the measuring capillary tube.
 8. A soil moisture contentmeasurement system as claimed in claim 1 wherein the measurementapparatus comprises a series of infrared emitter and infrared detectorpairs spaced along the measuring capillary tube.
 9. A method ofmeasuring a moisture retention curve of a soil sample comprising thesteps of. supporting a soil sample on a porous plate; positioning theheight of a measuring capillary tube with respect to the porous plate toenable liquid to be conveyed between the measuring capillary tube andthe porous plate; positioning a hanging water tube to convey liquidbetween the porous plate and the measuring capillary tube; performing atleast one purging cycle in which liquid is introduced into the measuringcapillary tube and the soil sample; performing at least one drying cyclein which the measuring capillary tube is substantially emptied ofliquid, liquid is permitted to travel from the soil sample through theporous plate to the measuring capillary tube, and the movement of liquidwithin the measuring capillary tube is measured; storing in computermemory data representing liquid movement measurements e measuringcapillary tube; and calculating the moisture retention curve from thedata representing liquid movement measurements.
 10. A method ofmeasuring a moisture retention curve as claimed in claim 9 furthercomprising the step of performing, after the or each drying cycle, oneor more wetting cycles in which liquid is permitted to travel from themeasuring capillary tube through the porous plate to the soil sample,and the movement of liquid within the measuring capillary tube ismeasured.
 11. A method of measuring a moisture retention curve asclaimed in claim 10 further comprising the step of periodicallyintroducing liquid into the measuring capillary tube once the tube issubstantially empty of liquid.
 12. A method of measuring a moistureretention curve as claimed in claim 10 further comprising the step ofterminating the or each wetting cycle on detection of substantially nomovement of liquid within the measuring capillary tube during apredefined time limit.
 13. A method of measuring a moisture retentioncurve as claimed in claim 10 further comprising the step of terminatingthe or each wetting cycle on user input.
 14. A method of measuring amoisture retention curve as claimed in claim 9 further comprising thestep of periodically substantially emptying the measuring capillary tubeof liquid during the drying cycle once the volume of liquid within themeasuring capillary tube reaches a predefined maximum.
 15. A method ofmeasuring a moisture retention curve as claimed in claim 9 furthercomprising the step of terminating the or each drying cycle on detectionof substantially no movement of liquid within the measuring capillarytube during a predefined time limit.
 16. A method of measuring amoisture retention curve as claimed in claim 9 further comprising thestep of terminating the or each drying cycle on user input.